Between Race and South streets, the majority of street names come from trees: Cherry, Chestnut, Walnut, Locust, Spruce, and Pine all bear the names of trees. If you look at a map from before 1853, you'll find that Race, Arch, and South streets used to be called Sassafrass, Mulberry, and Cedar streets respectively. Most cities have an Oak Lane somewhere, but the profusion of streets named after trees in Philly's central grid is striking: most tourists and newcomers to the city notice it immediately.
Though it's not totally clear why the streets are named after trees, there are a few compelling theories. One is simply that William Penn loved nature, and, in keeping with his recorded commitment to making Philadelphia into a "greene countrie towne," he gave city streets the names of plants from the forest.
That's certainly part of the explanation, but lore also holds that naming streets after trees was savvy urban planning, at a time when most Philadelphians were illiterate. Instead of posting street signs, (which were completely useless to illiterate citizens,) William Penn decided to plant a certain type of tree down the side of each street, to indicate the name of the street (Chestnut trees planted on Chestnut street, and so on.) That way, illiterate citizens could navigate the city.
Incidentally, lore also holds that Race street, which was originally known as Sassafrass street earned its new name by playing host to 18th century drag races. You can still see a street marker bearing the old street name at the southwest corner of Race and 3rd.
· What's in an Odanym? [Hidden City]
· Philadelphia Streets [Philly Trees]